version 3.0 [rewritten]

CloremDB/build.gradle

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+plugins {
+    id 'java-library'
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_16
+    targetCompatibility = JavaVersion.VERSION_16
+}
+
+dependencies {
+    implementation 'androidx.annotation:annotation:1.3.0'
+}

CloremDB/src/main/java/db/clorabase/clorem/Clorem.java

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+package db.clorabase.clorem;
+
+import androidx.annotation.NonNull;
+
+import java.io.File;
+import java.lang.management.ThreadInfo;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.Callable;
+import java.util.function.Consumer;
+import java.util.function.Predicate;
+
+import javax.naming.NoPermissionException;
+
+/**
+ * Clorem is a simple, easy to use, and fast database engine for java applications.
+ * It is a no-sql database which can save POJO's directly. You can organise your data
+ * in volumes. Each volume is like a SQL table which contains the objects as rows. You can create
+ * as many volumes as you want. You can also create as many objects as you want in
+ * each volume. The only limitation is that the objects implement the {@link CloremObject}
+ *
+ * All the methods are thread safe. You can use the database in a multi-threaded environment.
+ *
+ * @author author Rahil khan
+ * @version 1.0
+ * @since 1.0
+ */
+public class Clorem {
+    private static Clorem INSTANCE;
+    private static File databaseDir;
+
+    /**
+     * This method returns the singleton instance of the Clorem database.
+     * @param databaseDir The directory where the database will be stored on the disk. This directory must be writable.
+     * @return The singleton instance of the Clorem database
+     */
+    public static Clorem getInstance(File databaseDir) {
+        if (INSTANCE == null){
+            INSTANCE = new Clorem();
+            if (databaseDir.canRead() && databaseDir.canWrite())
+                Clorem.databaseDir = databaseDir;
+            else
+                throw new RuntimeException(new NoPermissionException("You do not have permission to read/write on this directory"));
+        }
+        return INSTANCE;
+    }
+
+    /**
+     * This method creates a new volume in the database.
+     * @param name The name of the volume
+     */
+    public void createVolume(@NonNull String name){
+        new File(databaseDir,name).mkdir();
+    }
+
+    /**
+     * This method deletes a volume from the database. All the objects in the volume will be deleted.
+     * @param name The name of the volume
+     */
+    public void deleteVolume(@NonNull String name){
+        var volume = new File(databaseDir,name);
+        var files = volume.listFiles();
+        for (File file : files) {
+            file.delete();
+        }
+        volume.delete();
+    }
+
+    /**
+     * Put an object in the database. The object must implement the {@link CloremObject} interface.
+     * @param object The object to be saved
+     * @param <T> The type of the object
+     */
+    public synchronized <T extends CloremObject> void put(@NonNull T object){
+        var volume = new File(databaseDir,object.getVolume());
+        if (!volume.exists() && !volume.mkdir())
+            throw new RuntimeException("The volume could not be created. You might not have permission to create a directory");
+        else
+            CloremUtils.writeObject(object,new File(volume,object.getKey()));
+    }
+
+    /**
+     * Get an object from the database.
+     * @param key The key of the object
+     * @param volume The volume in which the object is stored
+     * @return The object
+     */
+    public synchronized Object get(@NonNull String key,@NonNull String volume){
+        var volumeDir = new File(databaseDir,volume);
+        var obj = new File(volumeDir,key);
+        if (volumeDir.isDirectory() && obj.exists())
+            return CloremUtils.readObject(obj);
+        else
+            throw new IllegalArgumentException("The volume or the object does not exist");
+    }
+
+    /**
+     * Update an object in the database. Only use this method when you want to update whole object.
+     * If you want to update only a few fields of the object, use the {@link #update(String, String, Class, Consumer)} method.
+     * The object id must be the same as the object id of the object in the database.
+     * @param object The object to be updated
+     * @param <T> The type of the object
+     */
+    public synchronized <T extends CloremObject> void update(@NonNull T object){
+        var volume = new File(databaseDir,object.getVolume());
+        var file = new File(volume,object.getKey());
+        if (volume.isDirectory() && file.exists()){
+            CloremUtils.writeObject(object,file);
+        } else
+            throw new IllegalArgumentException("The volume or the object does not exist");
+    }
+
+    /**
+     * Update an object in the database. This method is used to update only required fields of the object.
+     * @param key The key of the object
+     * @param volume The volume in which the object is stored
+     * @param clazz The class of the object
+     * @param transition The transition function which will be used to update the object
+     * @param <T> The type of the object
+     */
+    public synchronized <T extends CloremObject> void update(@NonNull String key, @NonNull String volume,@NonNull Class<T> clazz,@NonNull Consumer<T> transition){
+        var obj = clazz.cast(get(key,volume));
+        transition.accept(obj);
+        if (key.equals(obj.getKey()) && volume.equals(obj.getVolume()))
+            update(obj);
+        else {
+            delete(key,volume);
+            put(obj);
+        }
+    }
+
+    /**
+     * Delete an object from the database.
+     * @param key The key of the object
+     * @param volume The volume in which the object is stored
+     */
+    public synchronized void delete(@NonNull String key,@NonNull String volume){
+        var volumeDir = new File(databaseDir,volume);
+        var obj = new File(volumeDir,key);
+        if (volumeDir.isDirectory() && obj.exists())
+            obj.delete();
+        else
+            throw new IllegalArgumentException("The volume or the object does not exist");
+    }
+
+    /**
+     * Query the database. This method returns a list of objects which satisfy the condition.
+     * @param volume The volume in which the objects are stored
+     * @param clazz The class of the objects
+     * @param condition The condition which the objects must satisfy. Returns true if the object is to be included in the list.
+     * @param <T> The type of the objects
+     * @return A list of objects which satisfy the condition
+     */
+    public <T extends CloremObject> List<T> query(@NonNull String volume,@NonNull Class<T> clazz, @NonNull Predicate<T> condition){
+        var files = new File(databaseDir,volume).listFiles();
+        if (files == null)
+            throw new RuntimeException("The volume does not exist");
+        else {
+            var list = new ArrayList<T>();
+            for (File file : files) {
+                var obj = get(file.getName(),volume);
+                try {
+                    if (condition.test(clazz.cast(obj)))
+                        list.add(clazz.cast(obj));
+                } catch (ClassCastException ignored){}
+            }
+            return list;
+        }
+    }
+
+    /**
+     * List all the objects in a volume.
+     * @param volume The volume in which the objects are stored
+     * @return A list of objects
+     */
+    public List<Object> listObjects(@NonNull String volume){
+        return CloremUtils.listObjects(new File(databaseDir,volume));
+    }
+}

CloremDB/src/main/java/db/clorabase/clorem/CloremObject.java

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+package db.clorabase.clorem;
+
+import java.io.Serializable;
+
+/**
+ * This interface is implemented by all the objects which are to be saved in the database.
+ * The objects must implement this interface to be saved in the database.
+ *
+ * @author author Rahil khan
+ * @version 1.0
+ * @since 1.0
+ */
+public interface CloremObject extends Serializable {
+
+    String getKey();
+    String getVolume();
+}

CloremDB/src/main/java/db/clorabase/clorem/CloremUtils.java

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+package db.clorabase.clorem;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.FileNotFoundException;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.UncheckedIOException;
+import java.util.Arrays;
+import java.util.List;
+
+public class CloremUtils {
+
+    protected static void writeObject(Object object, File file){
+        try {
+            var os = new ObjectOutputStream(new FileOutputStream(file));
+            os.writeObject(object);
+            os.close();
+        } catch (IOException e) {
+            throw new UncheckedIOException(e);
+        }
+
+    }
+
+    protected static Object readObject(File file){
+        try {
+            var in = new ObjectInputStream(new FileInputStream(file));
+            var object = in.readObject();
+            in.close();
+            return object;
+        } catch (IOException | ClassNotFoundException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    protected static List<Object> listObjects(File volume){
+        var files = volume.listFiles();
+        if (files == null)
+            throw new RuntimeException("The volume does not exist");
+        else {
+            var objects = new Object[files.length];
+            for (int i = 0; i < files.length; i++) {
+                objects[i] = readObject(files[i]);
+            }
+            return Arrays.asList(objects);
+        }
+    }
+}